Load bearing structure and method of manufacture thereof

ABSTRACT

A load bearing structure configured to bear a load, the structure comprising a multiple cells. The load bearing structure has a length, a center, and a cell density, which varies at least along the length of the load bearing structure, which weighs at least 6 kg.

BACKGROUND

This invention relates generally to load bearing structures, andspecifically, to efficient load bearing structures and methods formanufacturing the same.

Load bearing structures such as railway ties or railway sleepers serveto transfer the rail loading from the wheel load, which is around 500kN, to support the structure of the train and the railroad base, tofacilitate in gauge maintenance, and to absorb vibrations imparted tothe railway tracks, among other functions. Popular conventionalmaterials for railway sleepers include concrete, steel and wood.Concrete is typically a very rigid material and therefore has poor shockabsorption characteristics, while steel also suffers from poor vibrationabsorption characteristics, while use of wood is increasingly beingdiscouraged because it results in depletion of natural resources. Infact, in many countries, policies discontinuing the use of woodensleepers have been affected. Accordingly, there is a need in theindustry for an alternative material for railway sleeper due to problemswith conventional sleepers. For instance, polymeric railroad sleepershave emerged as a probable alternative. Some contemplated polymericrailway sleepers include recycled, reinforced plastic constituents,sandwich and hybrid concepts. These conceptualizations, however, sufferfrom a number of disadvantages, such as high weight, leading to anincrease in material costs, and low strength to weight ratios, amongothers.

Further, the weight of the conventional railway sleepers ranges from 100kg to 200 kg, and in general there exists a need for railway sleeperswith reduced weight. There is also a need to improve the load bearingcapacity, gauge maintenance and vibration characteristics of thealternate material railroad sleepers, and a need for efficientlymanufacturing such railway sleepers.

BRIEF DESCRIPTION

Briefly, in accordance with one embodiment of the invention, there isprovided a load bearing structure. The load bearing structure isconfigured to bear a load, the load bearing structure includes a numberof cells and the load bearing structure has a length, a center, and acell density. The cell density of the load bearing structure variesalong the length. The load bearing structure has a mass of 6 kg or more.

In accordance with another embodiment of the invention, there isprovided a method for manufacturing a load bearing structure. The methodincludes providing a mold configured for making the load bearingstructure, injection molding a suitable composite material into themold, and recovering the injection-molded load bearing structure.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a front elevation perspective view of a load bearing structurein accordance with an embodiment of the invention.

FIG. 2 is a top perspective view of the load bearing structure of FIG.1.

FIG. 3 is a bottom perspective view of the load bearing structure ofFIG. 1.

FIG. 4 is a plot illustrating a load on the load bearing structure andcell density versus the length of the load bearing structure of FIG. 1.

FIG. 5 is front view of a load bearing structure with consolidated partsin accordance with another embodiment of the invention.

FIG. 6 illustrates a method for manufacturing a load bearing structurein accordance with an embodiment of the invention.

DETAILED DESCRIPTION

As noted, the present invention provides a load bearing structure forbearing a load. FIGS. 1-3 illustrate such a load bearing structure, forexample, a railway sleeper 10 in accordance with one embodiment of theinvention. The railway sleeper 10 holds and supports the rails, bearinga load P at localized zones in the railway sleeper, illustrated in FIG.1 as equally distributed (P/2 for each rail). The load P typicallyrepresents the load of railway vehicles imparted to the sleeper 10,typically through two rails (not shown). The railway sleeper 10 may besupported by girders 12 as shown, or other alternate supports such asthose employed in ballast tracks, or other alternate supports such asthose employed in ballastless tracks may be used. Critical parametersfor railway sleeper performance include load bearing capacity or bearingstiffness, and dimensional stability or gauge maintenance. Importantly,vibration absorption characteristics of the railroad sleeper 10translate into passenger comfort through reduction in noise andvibrations. Other important considerations include the weight of therailway sleeper, its cost, and the cost and complexity of additionalfixtures required for facilitating load bearing, such as rail fasteners.An aspect of the invention resides in providing a railway sleeper or aload bearing structure that offers improvement in these parameters.

According to an inventive aspect of the invention, the load bearingstructure is non-uniformly composed to efficiently support the load.Specifically, the load bearing structure comprises multiple cells 14(FIG. 3) having walls, within the railway sleeper 10 body, the cells 14configured to bear loads according to the spatial loading requirementsof the railway sleeper 10. The cells 14 may proceed from top of thesleeper 10 to the bottom (along the height) or from left of the sleeper10 to the right (along the width). More specifically, it was observedthat the key loading for railway sleepers comes from the rail loading,as illustrated by loads P/2 shown in FIG. 1. Accordingly, the cells 14are distributed based on the expected loading characteristics of theload bearing structure 10. For example, the invention advantageouslyprovides a high density of cells 16 immediately below the zone of loadtransfer from the rail to the sleeper. A reduced cell density 18 may beemployed at locations distant from the primary load transfer zones. Byspatially positioning the cells 14, the load bearing capacity of therailway sleeper is concentrated in the zones where the loading is high,and this concept is employed for both transverse and longitudinal loadbearing capacity of the railway sleeper. It is noted here that the term“spatially varying” includes variations along the length, breadth andheight of the load bearing structure. By concentrating material where itis required, the present invention performs the intended load bearingfunction at about a third of the material weight in some cases. Thus,the load bearing structure provided by the present invention offerssignificant weight and cost reductions over conventional load bearingstructures.

In an embodiment, the cell density is configured to vary according tothe loading of the load bearing structure 10. Such a configuration isillustrated by the plot of FIG. 4, wherein the cell density 20 is at amaximum at the locations of concentrated loading, illustrated by curves30, for the embodiment of FIGS. 1-3. In another embodiment, the celldensity varies symmetrically from the center of the load bearingstructure, and in another embodiment the cell density variesun-symmetrically from the center. Also, in certain embodiments, thecells 14 may have a closed configuration with walls enclosing a volumefrom all sides, as opposed to an open configuration of illustratedembodiment of FIGS. 1-3, in which the cells have at least one open sidewithout a wall. It is appreciated that the load bearing structures beingdiscussed have a load bearing capacity above about 10 kN, for examplesupport beams, and railway sleepers 10 for which typical loads are about200-500 kN.

In addition to performing the critical functions such as load bearing,gauge maintenance, weight and cost reductions, the material constituentsfor the railway sleeper may also be tailored for desired features. In anembodiment, rail fixtures such as sloped top surface, transverse railsupports, rail fasteners, bolt holders among others, may be integratedin the sleeper, thereby beneficially providing a consolidated partsfeature. Parts consolidated at the time of manufacture advantageouslyeliminate the need to attach or fix those parts when the load bearingstructure is put in service, reducing the need for labor and equipmentat that time.

In an embodiment the load bearing structure, such as the railway sleeper10, comprises a polymeric material. Polymeric materials includethermoplastics and thermosets, and combinations thereof. Morespecifically polymeric materials suitable for use in the load bearingstructures of the present invention may be selected from materials suchas polycarbonates, polyamides, olefin polymers, polyesters,polyestercarbonates, epoxides, polysulfones, polyethers,polyetherimides, polyimides, silicone polymers, phenol formaldehyderesins, mixtures of the foregoing polymers, copolymers of the foregoingpolymers, and mixtures thereof.

In another embodiment, the load bearing structure comprises a compositematerial. The composite material typically includes an organic polymericmatrix with a filler material dispersed in the organic polymer matrix.Suitable materials for use as the organic polymeric matrix includethermoplastics, thermosets and combinations thereof. The organicpolymeric matrix and the filler material are chosen to impart desiredproperties to the load bearing structure, such as decreased thermaldimensional variation (thermal expansion or contraction), high bearingstrength, rigidity, and vibration damping characteristics, among others.Suitable filler materials include glass fibers, carbon fibers, polymericfibers, natural fibers, and zeolites, among others. The load bearingstructure may further be configured to comprise functional surfaces suchas surfaces comprising a weatherable coating layer, chemical resistancecoating, surfaces comprising an anti algae coating, surfaces comprisingan anti slip coating, and combinations thereof. In other embodiments,the load bearing structure's polymeric material or filler material mayimpart the above functionalities.

Embodiments of the present invention utilizing surfaces and cells forperformance enhancement and spatially varying cell densityconfigurations in load bearing structure 10 have been described. Theinvention however is useful in other alternative configurations as well.For example, in an embodiment of the invention, the spatially varyingcell density configuration can be custom designed for girder bridgesupports. In yet another embodiment of the invention, the spatiallyvarying cell density configuration can be custom designed for use incombination with ballasted tracks. In yet another embodiment of theinvention, the spatially varying cell density configuration can becustom designed for use in combination with ballastless tracks. In oneembodiment of the present invention, a load bearing structure comprisesat least one organic polymeric matrix material and at least tworeinforcement filler materials wherein a first reinforcement fillermaterial has a negative thermal expansion coefficient (for examplecarbon fibers) and a second reinforcement filler has a positive thermalexpansion coefficient (for example glass fibers). The two reinforcementfillers are present in amounts such that the overall thermal expansioncoefficient is zero. The use of fillers having offsetting thermalexpansion characteristics is useful in controlling the dimensionalintegrity of a structure, for example gauge maintenance in railroadapplications.

FIG. 6 illustrates a method 100 for manufacturing a load bearingstructure in accordance with an embodiment of the invention. In step110, a mold having cavities configured to form the load bearingstructure, the structure comprising cells is provided. In step 120, asuitable polymeric material is injected into the mold. The polymericmaterial may be a single polymeric material, a mixture of polymericmaterials, or a composite material. In step 130, the load bearingstructure is recovered from the mold. It is appreciated here thatvarious injection molding techniques, or other techniques such as highpressure plastic injection molding, high or low pressure structural foammolding, gas assist injection molding, extrusion, thermoset molding,injection-compression molding, water assist molding, multi shot moldingare generally known in the art, and any of such obvious techniques maybe used without deviating from the scope and spirit of the invention. Inmulti-shot injection molding, the railway sleeper is molded in two ormore injection shots of the polymeric material. For example, a left halfof the part along length is first molded, and then a right half ismolded to complete the sleeper. In another embodiment, the sleeper maybe manufactured in different parts. These different parts are moldedseparately, and then joined together using mechanical type of joints forexample, Dovetail joint or other fastening techniques known in the art,for example thermoplastic welding, bolt and screws, among others. In oneembodiment, the railway sleeper 10 comprises an open cell configuration(or “rib cell”) as illustrated in FIG. 3 that meets the engineeringrequirements of a load bearing structure such as a railway sleeper andmay be manufactured by an injection molding process and provides forfaster cycle time and more cost effective fabrication relative to themanufacture of structures lacking the open cell configuration. In anembodiment, functional parts, such as those not required for the purposeof bearing the load, for example rail fasteners, rail supports, boltholders, among others, may be advantageously co-molded into the loadbearing structure 10. For example, FIG. 5 illustrates a railway sleeper10 having consolidated rail fasteners 22.

Numerical Evaluation Section

A comparison of performance parameters of a conventional polymer railwaysleeper (Comparative Example), and a railway sleeper comprisingspatially varying cell density structure (Example 1), indicates theadvantages brought forth by various embodiments discussed above. Thefollowing data was generated by simulating various Railway sleepers byforming a test mesh using Hypermesh™ software, and testing it forstrength (maximum Von-Mises stress and maximum deflection) using ABAQUS™software, and for manufacturing (Injection mold, moldability,productivity and shot capacity of machine) using Moldflow™ software. Theterm “example” as used herein will be understood to refer to a numericalsimulation outcome, and not an actual physical test.

As can be seen from Table 1, the part volume and part weight valuesrequired to meet the engineering requirements for the load bearingstructure of the Comparative Example as compared to Example 1 indicatesa substantially higher volume and mass of material is required for apolymeric railway sleeper having a conventional design relative to thesleeper design of Example 1, which represents polymeric railway sleeperspossessing spatially varying cell density.

The railway sleeper of Example 1 comprising a rib cell structureadvantageously provides for an injection molding process that is asimple manufacturing process, simple injection molds, low tooling cost,easy moldability, high productivity and lower shot capacity machine, ascompared to that of the Comparative Example. The volume and weightreduction in Example 1 is substantial, and accordingly the reduction inmaterial costs is also substantial. TABLE 1 Parameter Comparativeexample Example 1 Part Volume 30700 cm3 16320 cm3 Part Weight at densityof 1.5 46 kg 24 kg gm/cm3 Maximum Von-Mises stress 56.48 N/mm² 36.16N/mm² Maximum deflection 0.65 mm 0.52 mm Shot capacity of machine =36840 cm3 19584 cm3 Minimum 1.2 × Part volume

As can be seen from Table 1, the high part volume, and accordingly highpart weight values are required to meet the engineering requirements forthe load bearing structure of the Comparative Example as compared toExample 1, which is polymeric railway sleepers possessing spatiallyvarying cell density. The railway sleeper of Example 1 comprising a ribcell structure provides for an advantageous injection molding process,requiring a low shot capacity of injection molding machine for Example1, as compared to that of the Comparative Example. The advantage ofweight reduction exhibited by Example 1 is substantial, as is theassociated cost reduction. The weight reduction of Example 1 incomparison to conventional sleepers, such as those of wood is especiallyadvantageous. A low von-Mises stress and a low deflection of the Example1 in comparison to the conventional polymeric sleeper indicates a higherstability and toughness of the Example 1, and are further advantagesexplored by the present design provided by the invention. Whilepreserving desired strength, and other desirable parameters such asvibration damping, the railway sleeper of Example 1 weighs about 24kilograms, whereas conventional sleepers may weigh between 100 and 200kilograms, and current polymeric sleepers may weigh up to about 50kilograms. Additionally, ease of manufacture, which is an importantfactor, is also an advantageous aspect of the embodiment of Example 1.This is also illustrated by nearly half the shot capacity required formanufacturing the conventional polymeric sleeper. Other advantagespertaining to manufacture include use of injection molding process,simple injection molds, low tooling costs, high moldability andproductivity as compared to the conventional polymeric sleeper.Therefore, the configurations illustrated in Example 1, as projected andcompared with the Comparative Example are a substantial improvement overthe existing art.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood by thoseskilled in the art, that variations and modifications can be effectedwithin the spirit and scope of the invention.

1. A load bearing structure configured to bear a load, said load bearingstructure comprising a plurality of cells, said load bearing structurehaving a length, a center, and a cell density, wherein said cell densityvaries at least along said length, and said load bearing structurehaving a mass of at least 6 kg.
 2. The load bearing structure of claim1, wherein said plurality of cells comprises open cells.
 3. The loadbearing structure of claim 1, wherein said cell density variessymmetrically from the center of said load bearing structure.
 4. Theload bearing structure of claim 3, wherein said cell density isconcentrated in a region bearing a substantial component of said load.5. The load bearing structure of claim 1, wherein said cell densityvaries unsymmetrically from the center of said load bearing structure.6. The load bearing structure of claim 1, wherein said plurality ofcells comprises at least one closed cell.
 7. The load bearing structureof claim 14, wherein said load is greater than about 10 kN.
 8. The loadbearing structure of claim 1, which is a support beam.
 9. The loadbearing structure of claim 1, which is a railway sleeper.
 10. The loadbearing structure of claim 9, further comprising a consolidated part.11. The load bearing structure of claim 9, wherein the consolidated partis a rail support mechanism.
 12. The load bearing structure of claim 11,wherein said rail support mechanism is a support structure attachmentmechanism.
 13. The load bearing structure of claim 1 comprising at leastone polymeric material
 14. The load bearing structure of claim 13,wherein said polymeric material is selected from the group consisting ofthemoplastics, thermosets and combinations thereof.
 15. The load bearingstructure of claim 14, wherein said polymeric material is selected fromthe group consisting of polycarbonates, polyamides, olefin polymers,polyesters, polyestercarbonates, epoxides, polysulfones, polyethers,polyetherimides, polyimides, silicone polymers, mixtures of theforegoing polymers, copolymers of the foregoing polymers, and mixturesthereof.
 16. The load bearing structure of claim 1 comprising at leastone composite material.
 17. The load bearing structure of claim 16,wherein said composite material comprises an organic polymeric matrixand a filler material dispersed therein.
 18. The load bearing structureof claim 17, wherein the filler material is a glass fiber.
 19. The loadbearing structure of claim 17, wherein the filler material is a carbonfiber.
 20. The load bearing structure of claim 17, wherein the fillermaterial is a polymeric fiber.
 21. The load bearing structure of claim17, wherein the filler material is a natural fiber.
 22. The load bearingstructure of claim 17, wherein the organic polymeric matrix is selectedform the group consisting of thermoplastics, thermosets and combinationsthereof.
 23. The load bearing structure of claim 17, wherein the fillermaterial is configured to decrease dimensional variation.
 24. The loadbearing structure of claim 23, wherein the filler material is selectedfrom the group consisting of carbon fibres and zeolites.
 25. The loadbearing structure of claim 17, wherein the load bearing structurecomprises functional surfaces.
 26. The load bearing structure of claim25, wherein said functional surfaces comprises a coating selected fromthe group consisting of weatherable coating, chemical resistance coatinganti algae coating, anti slip coating and combinations thereof.
 27. Theload bearing structure of claim 9, wherein the railway sleeper isconfigured to be compatible with a girder bridge, a ballasted track or aballast less track.
 28. A method of manufacturing a load bearingstructure, the method comprising: providing a mold having cavitiescomplimentary to the load bearing structure; molding a polymericmaterial into the mold; and recovering the load bearing structure fromthe mold, wherein the load bearing structure configured to bear a load,the load bearing structure comprising a plurality of cells, the loadbearing structure having a length, a center, and a cell density, whereinsaid cell density varies at least along said length, and the loadbearing structure having a mass of at least 6 kg.
 29. The method ofclaim 28, wherein said molding comprises one selected from the groupconsisting of high pressure plastic injection molding, high pressurestructural foam molding, low pressure structural foam molding, gasassist injection molding, extrusion, thermoset molding,injection-compression molding, water assist molding and combinationsthereof.
 30. The method of claim 28, wherein the mold has cavitiescomplimentary to a rib cell structure.
 31. The method of claim 28,further comprising co-molding a functional part into the load bearingstructure.
 32. The method of claim 28, further comprisingsurface-treating the load bearing structure.
 33. The method of claim 32,wherein said surface treating is selected from the group consisting ofpainting, plating, electrolytic coating, spraying and combinationsthereof.